A gas turbine is a continuous-flow machine in which a pressurized gas expands. It is composed of a turbine or expander, a compressor connected upstream and an intermediately connected combustion chamber. The principle of action is based on the cyclic process (Joule process): the latter compresses air by means of the blades of one or more compressor stages, subsequently mixes this in the combustion chamber with a gaseous or fluid propellant, and ignites and burns it.
This results in a hot gas (a mixture of combustion gas and air) which relaxes in the downstream turbine part, wherein the thermal energy is converted into mechanical energy and firstly drives the compressor. The remaining portion is used in the turboshaft engine to drive a generator, a propeller or other rotating consumers. In contrast, in the case of a jet engine the thermal energy accelerates the hot stream of gas, which generates the thrust.
The regulation of the fuel quantity of a fixed gas turbine during operation is usually carried out by means of a target value for the corrected turbine outlet temperature which is determined from the actual, measured outlet temperature. The object here is to keep the turbine inlet temperature largely constant as an important design variable of the gas turbine even under changing ambient conditions.
However, in the case of relatively low ambient temperatures and therefore relatively cool compressor air, the turbine inlet temperature and therefore also the turbine outlet temperature drop. The specified regulating process therefore increases the fuel quantity, as a result of which the primary zone temperature/combustion temperature in the combustion chamber increases. In the case of low external temperatures, this can cause this temperature to rise so far that it has a negative effect on the combustion stability. As a result, faults can occur during operation of the gas turbine.
In most cases, this problem has been solved by virtue of the fact that a characteristic curve (Cold Ambient Curve, CAC) was stored in the control unit of the gas turbine, said curve limiting or lowering the target value for the corrected turbine outlet temperature in the case of cold ambient temperatures, with the result that the rise in temperature in the combustion chamber is also limited. However, since as a result of this the turbine is no longer operated at the design point, the performance and efficiency of the turbine are reduced.
However, according to the teaching of EP2239641A2 there can also be provision to determine a target value for the corrected turbine outlet temperature by means of a logic circuit, wherein in order to calculate the decisive switching criterion a parameter which is characteristic of the combustion stability is taken into account. However, the switching logic requires continuous re-calculation of possibly different target values from which just one is subsequently used to control the gas turbine. This is very costly in terms of calculation technology since, in particular in the case of open-loop control, the possibility of combustion instability is continuously taken into account even in operating states of the gas turbine in which no combustion instability at all can occur owing to the operating conditions.